Method of breaking a brittle substrate

ABSTRACT

The invention relates to a method of breaking a substrate of a brittle material, the method comprising the steps of providing a substrate ( 1 ) of a brittle material, heating the substrate with a laser beam ( 3 ) to create a heated spot on the substrate, moving the laser beam and the substrate with respect to each other to create a line of heated spots on the substrate ( 2 ), cooling the heated spots on the substrate by locally applying a cooling medium ( 4 ) behind the heated spots such that a micro-crack is propagated in the line of heated spots, and breaking the substrate along the line of the propagated micro-cracks by applying a mechanical force on the substrate wherein, the cooling medium comprises an aqueous surfactant solution. The surfactants will connect to the broken siloxane bonds inside the surface cracks. Then recombination and healing of the broken siloxane bonds will not occur and the required breaking load will remain constant over time.

The invention relates to a method of breaking a substrate of brittlematerial, the method comprising the steps of providing a substrate of abrittle material, heating the substrate with a laser beam to create aheated spot on the substrate, moving the laser beam and the substratewith respect to each other to create a line of heated spots on thesubstrate, cooling the heated spots on the substrate by locally applyinga cooling medium such that a micro-crack in the line of heated spots ispropagated, and breaking the substrate along the line of micro-cracks byapplying a force on the substrate.

Many products made from a brittle non-metallic material, e.g. glass andsemi-conductor wafer materials, are formed by separating a piece, sheet,wafer, or panel into a number of smaller pieces of the desired size orsizes.

U.S. Pat. No. 6,252,197 discloses a method for physically separatingbrittle substrates by forming a micro-crack in the substrate andcontrollingly propagating the micro-crack. An initial mechanical orlaser scribing device forms a micro-crack in the substrate. A laser beamis applied onto the substrate on a separation line. A coolant streamcomprising a mixture of pressurized gas and water intersects with thetrailing edge of the laser beam. The temperature differential betweenthe heat affected zone of the substrate and the coolant streampropagates the micro-crack. By applying a mechanical load the substrateis subsequently broken into smaller pieces along the separation line,which may be further processed e.g. to produce display devices.

It is a problem connected to this method that the mechanical loadnecessary to further open the induced surface cracks increases withtime, i.e. the mechanical load is higher the longer the time elapsedbetween the moment of inducing micro-cracks and the final breaking ofthe substrate into smaller pieces. This is undesired since in a factoryit is often not possible to immediately break the substrates after theyhave been treated with the laser beam. In that case the subsequentbreaking of the substrate results in damage of edges and a lowerproduction yield.

It is an object of the invention to provide a breaking process in whichthe breaking load remains constant over time after the initiation ofmicro-cracks. To this end the method according to the invention ischaracterized in that the cooling medium comprises an aqueous surfactantsolution. The inventors have realized that the increase of the breakingload in the conventional process is due to locking or even healing ofthe micro-cracks. However, if the cooling medium comprises an aqueoussurfactant solution the surfactants will connect to the broken siloxanebonds inside the micro-cracks. Recombination and healing of the brokensiloxane bonds will not occur and the required breaking load will remainconstant over time. Furthermore, the surfactants will change (i.e.lower) the surface energy of the cracks. Consequently, the cracks willbe kept open and the load needed to open the cracks will be lowered.

The dependent claims describe advantageous embodiments of the invention.

These and other objects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 is a schematic view from above of an apparatus employing themethod according the invention, and

FIG. 2 shows results of load measurements for various cooling media andvarious elapsed times.

The figures are not drawn to scale. In the figures, like referencenumerals generally refer to like parts.

FIG. 1 is a schematic view from above of an apparatus employing themethod according the invention. A spot 3 of a laser beam is focussed onsubstrate 1 of a brittle material, e.g. glass, crystalline silica,ceramics or compositions thereof. The energy contained in the laser spot3 causes local heating of the substrate. A cooling medium from a nozzle4 in the vicinity of the laser spot 3 (in most cases positioned behindthe laser spot) cools the heated spot. This rapid temperaturedifferential causes a thermal shock and causes a pre-existingmicro-crack to propagate.

The substrate is moved with respect to the laser spot in a directionindicated by the arrow. Likewise, the laser beam may be moved withrespect to the substrate in a direction opposite to the arrow. As aresult of the relative movement the micro-crack is propagated along aseparation line 2.

Next, the substrate 1 can be broken along the separation line 2 in aconventional way by exerting a mechanical load force to the substrate.

It has been observed that the mechanical load required to break thesubstrate depends on the amount of time elapsed between the formation ofthe micro-cracks and the moment of applying the mechanical load. FIG. 2shows results of load measurements for various cooling media and variouselapsed times. The data are provided with bars indicating thestatistical spread on the measurements. Displayed is the requiredmechanical load (in Newton) for breaking the substrate in case of:

-   a spray mixture of air and ethanol as cooling medium, 30;-   cooling by a spray mixture of air and water, and immediately    followed by breaking, 40;-   cooling by a spray mixture of air and water, and breaking after 12    hours, 50;-   cooling by a spray mixture of air and an aqueous solution comprising    0.1% by weight of the surfactant cetyl trimethyl ammonium bromide    (CTAB) and immediate breaking, 60;-   cooling by a spray mixture of air and an solution of water    comprising 0.1% by weight of the surfactant cetyl trimethyl ammonium    bromide (CTAB) and breaking after 15 hours, 70.

For reasons of comparison the mechanical load for the conventionalmechanical scribe process is also indicated, 20.

From the data shown in FIG. 2 it can be concluded that the mechanicalload is reduced by almost a factor of 2 if CTAB is used and that thereduction remains if breaking occurs after 15 hours. Good process yieldwas obtained when the glass plates were further processed and LCD panelswere made from them.

CTAB is a compound belonging to the class of cationic surfactants, i.e.a surface active agent, a substance such as a detergent that reduces thesurface tension of a liquid. Good results were also obtained withcompounds belonging to the classes of non-ionic and anionic surfactants,such as octadecyl deca(ethyleenoxide) hydroxide or dodecylbenzenesulfonic acid sodium salt, respectively. All compounds have in commontheir capability of binding to the broken siloxane bonds (‘danglingbonds’) inside the micro-cracks.

Good results were obtained when the compounds were present in theaqueous solution in the range of 0.01 to 1% by weight.

In summary, the invention relates to a method of breaking a substrate ofbrittle material, the method comprising the steps of providing asubstrate I of a brittle material, heating the substrate with a laserbeam 3 and creating a heated spot on the substrate, moving the laserbeam and the substrate with respect to each other thus creating a lineof heated spots on the substrate 2, cooling the heated spots on thesubstrate by locally applying a cooling medium 4 behind the heated spotssuch that a micro-crack is propagated in the line of heated spots,breaking the substrate along the line of the propagated micro-cracks byapplying a mechanical force on the substrate wherein, the cooling mediumcomprises an aqueous surfactant solution.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of other elements orsteps than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.

1. A method of breaking a substrate of brittle material, the methodcomprising the steps of: providing a substrate of a brittle material,heating the substrate with a laser beam to create a heated spot on thesubstrate, moving the laser beam and the substrate with respect to eachother to create a line of heated spots on the substrate, cooling theheated spots on the substrate by locally applying a cooling medium suchthat a micro-crack in the line of heated spots is propagated, andbreaking the substrate along the line of the propagated micro-crack byapplying a force on the substrate wherein the cooling medium comprisesan aqueous surfactant solution.
 2. A method of breaking a substrate ofbrittle material according to claim 1, wherein the cooling mediumfurther comprises air mixed with the aqueous surfactant solution.
 3. Amethod of breaking a substrate of brittle material according to claim 1,wherein the concentration of the surfactant is in the range of 0.01 to1% of weight.
 4. A method of breaking a substrate of brittle materialaccording to claim 1, wherein the aqueous surfactant solution comprisesa cationic surfactant.
 5. A method of breaking a substrate of brittlematerial according to claim 3, wherein the cationic surfactant comprisescetyl trimethyl ammonium bromide (CTAB).
 6. A method of breaking asubstrate of brittle material according to claim 1, wherein the aqueoussurfactant solution comprises a nonionic surfactant.
 7. A method ofbreaking a substrate of brittle material according to claim 5, whereinthe nonionic surfactant comprises octadecyl deca(ethyleenoxide)hydroxide.
 8. A method of breaking a substrate of brittle materialaccording to claim 1, wherein the aqueous surfactant solution comprisesan anionic surfactant.
 9. A method of breaking a substrate of brittlematerial according to claim 7, wherein the anionic surfactant comprisesdodecylbenzene sulfonic acid sodium salt.
 10. A method of breaking asubstrate of brittle material according to claim 1, wherein the brittlematerial comprises glass, crystalline silica, ceramics or compositionsthereof.